1. Field of the Invention
The present invention generally relates to optical fiber network switching and, more particularly, to a switching method having improved accuracy in monitoring a network to determine when to switch.
2. Description of the Related Art
Synchronous optical networking (SONET) and synchronous digital hierarchy (SDH) are standardized optical digital transmission systems that are used, respectively, in North America and internationally. SONET networks typically use synchronous add/drop multiplexers (ADM) to add and/or drop asynchronous DS-n signals onto the links. The ADM devices also re-route signals to avoid faulty communication links. This is referred to as span and ring switching.
Span switching and ring switching are mechanisms to re-route traffic over optical networks. A ring is a network configuration that allows signal path redundancy between nodes on a network by interconnecting the nodes in a loop, or ring. Span switching is performed between two nodes to re-route working traffic over the protection channel along the same path in the event of a failure on the working channel. The failure may be due to a fiber cut of the working channel or signal degradation due to other equipment failure. The working traffic is placed on the protection channel by the transmitting stations, then re-routed to the protection fiber/ring at the receiver, thereby bypassing the failed fiber/equipment. Ring switching is performed between two nodes to re-route working traffic over the protection channel in the opposite direction of the working path. Ring switching is used in the event of a complete fiber cut in the transmission/fiber line. The working traffic from the failed fiber span is thus re-routed to the protection fiber span.
SONET generally specifies that the protection time (the time it takes to re-route client traffic after detection of a failure) must be 50 ms or less; that the number of network elements (Nodes) in the ring is 16 or fewer; and that the total fiber distance around the ring is 1200 km or less. However, factors exist which cause the detection and initiation of the switching time to exceed 50 ms. For example, the size of the network and the length of optical fiber traversed are factors which can cause the time to exceed 50 ms.
Examples of factors that induce switching are a loss of signal (LOS) or bit error rate (BER) that is too high (i.e., a BER threshold is exceeded). However, current systems do not distinguish between a BER that exceeds the threshold due to a true line failure or a BER transient that momentarily causes the BER value to exceed the BER threshold. Transient BERs are generated due to, for example, the addition of one or more channels to a sparsely channel-populated WDM/DWDM system. When the new channel is added, the preexisting channels may undergo some power fluctuation, which could give rise to those channels seeing fluctuating BERs. Another example which causes a transient BER is movement of an optical fiber by a technician.
Local exchange carriers (LECs), and long distance carriers and other telecom service providers require a minimum quality of service (QoS) level. Without knowledge of the actual maximum switching time to an alternate transmission path, it is difficult for an LEC to guarantee the initiation of a switch within 50 ms. Absent an ability to prove adequate uptime rates, a service provider is unable to provide QoS guarantees to its customers.